Just think of all the hatemongers in this country; seeing a Jewish person on the ticket will be a motivation to vote, if only to vote against the Jew. And while, granted, having a Jewish candidate might very well sway the vote of many other Jews towards him, the number of negative voters will almost certainly outweigh the positive ones.

Now I don't know anything about Mr. Lieberman's political views, other than that he's a democrat. But I do know that he's Jewish, and despite America's claims of being socially tolerant, being so is an automatic downvote in many people's books.